First of all, a frame structure and a resource structure of a wireless communications system will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a diagram illustrating a frame structure of a wireless communications system. As shown in FIG. 1, one frame includes ten subframes, each of which includes two slots. The time required to transmit one subframe will be referred to as a transmission time interval (hereinafter, referred to as “TTI”). For example, one subframe may be 1 ms, and one slot may be 0.5 ms.
One slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols. The OFDM symbols may be referred to as SC-FDMA symbols or symbol duration.
One slot includes seven or six OFDM symbols in accordance with a length of cyclic prefix (hereinafter, referred to as “CP”). A long term evolution (hereinafter, referred to as “LTE”) system includes a normal CP and an extended CP. One slot includes seven OFDM symbols if a normal CP is used, while one slot includes six OFDM symbols if an extended CP is used. The extended CP is used if delay spread is great.
FIG. 2 is a diagram illustrating a resource structure of one downlink slot. Referring to FIG. 2, one slot includes seven OFDM symbols. A resource element (RE) is a resource region that includes one OFDM symbol and one subcarrier, and a resource block (RB) is a resource region that includes a plurality of OFDM symbols and a plurality of subcarriers. For example, the resource block may include seven OFDM symbols in a time domain, and may include twelve subcarriers in a frequency domain. The number of resource blocks included in one slot may be determined depending on a downlink bandwidth.
At present, an LTE-A system of which standardization is in progress supports carrier aggregation. Carrier aggregation is intended to extend a bandwidth by grouping a plurality of carriers, thereby enhancing a data rate.
Multicarrier refers to all frequency bands used by a base station, and a component carrier means an element carrier constituting multicarrier. In other words, a plurality of component carriers constitute multicarrier through carrier aggregation.
The system may be configured to allow a plurality of component carriers to use either the same physical cell ID (hereinafter, referred to as “PCI”) or different PCIS.
In the LTE Rel-8, since PCI related operations are performed based on PCI detected from downlink component carriers connected at 1:1, ambiguousness or confusion of the PCI related operations does not occur. However, if carrier aggregation is used, ambiguousness or interference may occur in the PCI related operations depending on whether PCIs of the plurality of component carriers are configured equally or differently and in accordance with connection relation between downlink component carriers and uplink component carriers.